Natural killer cells (NK) are thought to play a significant role as a first line of defense against infections and as an alternate surveillance system against neoplasm. They may also be involved in immunoregulation, bone marrow graft rejection, and acquired immunodeficiency syndrome, thereby making the characterization of NK an important issue. The delineation of NK has been hindered by the heterogeneous nature of this lymphoid population. In this project, NK will be further characterized by studying the subsets that constitute this heterogeneity. Specifically, NK cells will be fractionated from selected NK-containing organs by biophysical and serological means to yield greatly enriched subpopulations representative of different differentiation stages; to characterize these subsets by in vitro (target binding, range and specificity, suppression and serological studies with anti-NK monoclonal antibodies (MAbs), and in vivo (antitumor and suppression) studies. Subsets will be used to generate further MAbs. As purified and characterized NK subsets become available, the culture-derived cells will be propogated and subsequently cloned to yield continuous lines bearing NK specificities. Cloned lines will be characterized using the same in vitro and in vivo criteria. Selected NK-like cell lines will be used in homing and in in vitro organ culture studies to establish any correlation between differentiation maturity and migration of NK to various lymphoid organs. NK-like cell linea (noninduced or induced, targetspecific, etc.) will be used in adoptive transfer experiments involving control and NK-depressed mice in two murine tumor model systems so as to establish the in vivo significance of NK. Secondarily, selected NK-like lines will be used as immunogens for MAbs generation. Screened MAbs will be used for further phenotypic characterization of NK subsets. It is hoped that this combination of different methodologies will yield valuable information towards understanding subsets which comprise NK. (LB)